This invention pertains to the field of metrology and, more precisely, to the measurement of the displacement velocity of objects animated by a vibratory movement.
For the measurement of the rapid movements of an object, it is known to use equipment employing a laser beam divided into a direct fixed beam and an indirect secondary beam illuminating the vibrating target and interfering in a holographic material.
As an example, U.S. Pat. 3,899,921 describes a system that uses a stationary hologram and is based on the principle of holography in averaged time. In order to augment the amplitude of the measurable vibrations (limited in principle by the averaged time technique), the invention described in this patent of the prior art concerns a device using an auxiliary system that enables compensation of a more or less large part of the displacement of the target. The reference system provides a cartography of the displacement of the object but does not make it possible to follow the temporal variation of this vibration. The reference system is not functional on, for example, objects that are displaced in a transitory manner.
The articles by R. K. Ing and J. P. Monchalin xe2x80x9cBroadband optical detection of ultrasound by two-wave mixing in a photorefractive crystalxe2x80x9d, Appl. Phys. Lett. 59 (1991) 3233 and A. Blouin and J. P. Monchalin xe2x80x9cDetection of ultrasonic motion of a scattering surface by two-wave mixing in a photorefractive GaAs crystalxe2x80x9d, Appl. Phys. Lett. 65 (1994) 932 disclose the use of a two-wave mixing system for the detection of ultrasonic vibrations.
The system described in this document of the prior art operates by measuring the displacement whereas our system measures the instantaneous velocity of the vibrating target. Furthermore, the system that is presented in the article does not measure displacements exceeding the wavelength (circa 0.5 xcexcm), whereas the invention is designed for the analysis of large displacements (greater than several xcexcm).
Another article, A. A. Kamshilin, E. V. Mokrushina xe2x80x9cPossible use of photorefractive crystals in holographic vibrometryxe2x80x9d, Sov. Tech. Phys. Lett. 12 (1986) 149 describes the use of a photorefractive crystal in a two-wave mixing system for detection of large-amplitude, high-frequency vibrations. The principle is in fact very close to that of averaged time holography. In this case, when the hologram moves approximately xcfx80, the hologram is obliterated and diffraction is not produced. Thus, a cartography of the lines results in which the phase shift is xcfx80.
The article by S. Breugnot, M. Defour, J.-P. Huignard xe2x80x9cPhotorefractive two-wave mixing: complex amplitudes solutions in the case of a weak signal beamxe2x80x9d Optics Commun. 134 (1997) 599 describes a process for detection of vibrations by means of a hetrodyne or homodyne detection system based on two-wave mixing. One of the base hypotheses is that the vibrations are rapid in relation to the response time and the hologram formed can be obliterated by a vibration of excessively large amplitude. In the case of velocity measurement by Doppler shift (heterodyne detection), the authors consider a continuous grid displacement in the same direction and not of extension limited around a mean position as in the case of the measurement of the vibrational velocity of an object as in the case of the invention.
Furthermore, the article by T. J. Hall, M. A. Fiddy, M. S. Ner xe2x80x9cDetector for an optical fiber acoustic sensor using dynamic interferometryxe2x80x9d Opt. Lett. 5 (1980) 485 discloses the use of a holographic device for the measurement of acoustic vibrations. The described device only functions at high frequencies with small-amplitude vibrations to avoid obliteration of the grid.
This equipment according to the state of the art is suitable for small-amplitude, high-frequency vibratory displacements. In contrast, the equipment according to the state of the art is not suitable for measuring the velocity of targets vibrating at low frequencies with possibly large amplitude movements.
Thus, it would be advantageous to provide for the measurement of the velocity of objects animated by a vibratory displacement around a mean position without contact and without prior preparation, and to thereby make it possible to establish, among other things, a spectrum of the vibration velocities of this object. It would also be advantageous to provide a device enabling measurement of vibrations with amplitudes on the order of and greater than the wavelength of the light employed, with the study frequencies belonging to the low-frequency domains (typically about 0 to about 10 kHz).
Another advantage would be to enable a measurement that is not sensitive to the surface state of the object and can, therefore, be implemented without prior surface treatment of the object (polishing).
The invention relates to a process for velocimetric measurement of a target including causing interference in a dynamic holographic material of a reference laser beam and a beam diffused by the target, and measuring variations in luminous intensity transmitted by the material with a photoelectric detector, wherein the target vibrates with a displacement greater than the wavelength and with an extension limited around a mean position.
The invention also relates to a velocimeter including a laser which produces a beam, a beam separator which divides the beam into a reference beam and a target beam, a target vibrating at a frequency lower than about 10 kHz with an amplitude greater than about 0.1 xcexcm around a mean position, a dynamic holographic material into which the reference and target beams are directed, and a photodetector which detects a luminous signal produced by the dynamic holographic material.